Multi-user downlink systems typically comprise a multi-antenna base station that transmits multiple signals to each of a plurality of single-antenna mobile terminals. Because the mobile terminals typically only have one antenna, the mobile terminals cannot take advantage of simple receive diversity processing techniques. To address this issue, a base station in a multi-user downlink system may use channel state information reported by the mobile terminals to generate additional signals that may be used by the mobile terminals to create a virtual diversity receiver at each mobile station, where the virtual receiver at each mobile station has multiple virtual antennas to enable the mobile station to use linear diversity processing techniques to improve performance. For example, the base station may use previously reported channel state information to estimate signals received by the mobile terminals, where the base station may then use the received signal estimates to generate additional signals for subsequent transmission to the mobile terminals. Upon receipt, the mobile terminals may then use the additional signals to simplify the signal processing required to determine the originally transmitted signals, e.g., by using simple algebraic techniques. Thus, each mobile terminal experiences a performance boost, which may translate into better coverage, higher bit rate, higher cell throughput, etc. “Multi-User ARQ” by P. Larsson and N. Johansson, Vehicular Technology Conference, May 2006, vol. 4, pp. 2052-2057 discloses an exemplary virtual diversity receiver.
For example, a virtual diversity system may include a base station that uses three antennas to transmit information symbols to three single-antenna mobile terminals. After receiving channel estimates representing the channel state information for each of the channels between each of the transmission antennas and each of the mobile terminals at each transmission time, the base station estimates the signals received by the mobile terminals, and combines complementary pairs of the estimated signals to generate three combined signals, which are subsequently transmitted to the mobile terminals during three subsequent transmission times. The mobile terminals then use the originally received signals and the received combined signals to create a simple system of three equations having three unknowns. Each mobile terminal may then algebraically solve the corresponding system of equations to detect the three information symbols originally transmitted to that mobile terminal.
Because the channel may have changed significantly by the time the base station transmits the additional signal(s), the channel state information used to implement virtual diversity reception is generally thought of as stale feedback. However, virtual diversity reception does not rely on channel correlations over time, and therefore effectively assumes the channel is uncorrelated. Thus, the multi-user nature of virtual diversity reception renders stale channel state feedback very useful. “Completely Stale Transmitter Channel State Information is Still Very Useful” by M. Maddah-Ali and D. Tse, Allerton Conference, October 2010 discloses additional details regarding the use of stale channel state information.
Conventional virtual diversity receivers operate in a symmetric fashion, where each mobile terminal acts and benefits equally. As a result, all mobile terminals receive all transmitted signals and feedback all associated channel estimates. While all mobile terminals associated with such symmetric resource sharing benefit equally, implementing symmetric virtual diversity reception may place an unnecessary burden on the required signal processing and signaling overhead. This is especially problematic when all mobile terminals do not require the same benefits and/or overall performance results. Thus, there remains a need for improving the implementation of virtual diversity reception.